Increases in childhood obesity are a public health concern worldwide. In the U.S., 17.6 percent of adolescents have high BMIs for their age. Obesity early in life is associated with poorer physical and mental health and social wellbeing, including respiratory problems, early onset of type 2 diabetes, depression, and social exclusion, and with adult obesity and increased risk of chronic disease later in life. This study explores a potentially important influence on obesity risks in adolescence that has not received sufficient attention in the literature: friendships. OBJECTIVES: Focusing on adolescents, the proposed research will: 1) determine how weight relates to friendship formation;2) explore the relationship between weight and the strength of their friendship in terms of closeness and duration;and 3) examine whether friends influence weight-related behaviors, weight change in each other over time, and mechanisms through which this influence takes place. An additional innovative approach of this application is to consider how the relationship between weight status and friendships differs between youths by gender, race/ethnicity, and immigrant status. DESIGN: This project will perform secondary data analyses using the National Longitudinal Study of Adolescent Health (Add Health), a nationally representative longitudinal survey of adolescents. The unique design of Add Health allows researchers to link respondents'records with those of their schoolmates so that the trajectories of weight and health behaviors of both can be considered within their family, school, and neighborhood contexts -an advantage over most studies, which rely on respondents'claims about their friends. Another advantage is the longitudinal collection of data, which allows us to follow adolescents over time. METHODS: The study develops a conceptual framework for understanding the relationships between friendships and weight status. Survey-adjusted multivariate linear and logistic regressions will be used to assess the importance of weight status for adolescents'friendship in high school. Respondent records are linked to examine differences in friendship reciprocation by weight status and to explore the patterns of influence between friends'weights and health behavior in high school and 7 years later. To the extent possible, researchers will employ longitudinal analyses and instrumental variables to correct biases of unobserved heterogeneity and reverse causality, to distinguish between homophily and influence, and to isolate pathways of causality. SIGNIFICANCE: Understanding the relationships between the social environment and obesity risks can inform research and interventions about the social aspects of health behavior and the potential for tapping social networks to improve health and healthy behaviors in young people. If weight is an important factor in the creation of friendships and friends engage in weight-related behaviors together, interventions that recognize and make use of these relationships may be more successful at bringing about behavior change. No study to date has provided such a comprehensive picture of adolescent obesity at the national level. PUBLIC HEALTH RELEVANCE: The prevalence of obesity among children poses a challenge to develop interventions that promote healthy body proportions and healthy behaviors, and understanding the influences of friendships is important for developing these interventions. The proposed study will further current understanding of the ways in which adolescents'obesity risks and friendships are intertwined. Our findings will inform research and policy about: (a) the extent to which children's health behavior and weight are shaped by friendships;and (b) the importance of incorporating friends and peers into interventions to prevent and reverse obesity among American youths.